I did not understand how the digit 5 came in the answer. In the book it is writen that greedy quantifiers scan the entire source data and then they move backwards finding the appropriate match. I always get confused in how it does that !

we need to find a match according to the pattern [a-f]\d+ i.e, an alphabet ranging from a to f , then one or more(+ quantifier) digits in a row.
one such match is found at position 1 (b34) and the next at position 5(f0) , and hence the output 1b34 5f0

The pattern will match a sequence of characters which consists of exactly one occurence of characters a, b, c, d, e or f, followed by at least one digit.
Now, given the Stringab34ef0, which sub-sequences match this pattern?

Let's start at index 0 and work our way thru the sequence:

0 - No match here, a is a matching character, but it should be followed by one or more digits, and b certainly isn't that.

1 - Found a match! b is a matching cahracter, followed by 3 which is a digit! So are we done with this match? Not quite, because the greedy + quantifier will try to match as much of the sequence as it can, and the next character in sequence is 4, which is also a digit. Now we're done with this match, because the next character in sequence is e, which would break the pattern. Right, so now we print the starting position of this match (Matcher.start()) 1 and the match itself (Matcher.group()) b34 to the console, separated by a single white space, and we don't add a line separator, because we made a call to print(), not println().

4 - Wait a minute, isn't 1 ususally followed by 2? Well yes, but the previous match has 'consumed' the sequence up to and including the index position where that match ended: 3. Ok, so starting at position 4 we find e followed by f which doesn't match the pattern.

5 - Found another one! The sub-sequence f0 matches the pattern quite nicely. So lets print that to the console as well: 5 f0. Now, because we're using print() instead of println the output will be appended directly to the previous output.

Now we're done - the entire sequence has been consumed - and the output reads 1 b345 f0.
And that's the way the cookie crumbles

Edit: Oh crud, typing up my reply took longer than I thought, and the question has been answered in the mean time. Oh well...

Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life.

Read the chapter in K&B carefully it says that a quantifier will check it for the position after the last character in the string. That is the position next to last character and in this case it matches the pattern to me matched which is 0 or more .